Effects Of SL On Endogenous Hormone Metabolism And Related Gene Expression In Tobacco Axillary Bud

Strigolactone（SL）,a new hormone that regulates plant axillary bud growth and development,and has been one of the hot topics of researches in recent years,but it is still not clear how SL affects endogenous hormone metabolism and the expression of its related genes in axillary bud.In this study,we investigated the changes of tobacco axillary meristems development by using paraffin sectioning technique,and studied the effects of SL on endogenous hormone metabolism and hormone-related genes expression in tobacco axillary bud using high performance liquid chromatography-mass spectrometry,transcriptome analysis and fluorescence quantification techniques,in order to lay a theoretical foundation for the hormonal regulation mechanism of SL in regulating tobacco axillary bud outgrowth.The results showed that:（1）Tobacco axillary meristem consists of a central mother cell zone（CM）,a peripheral meristem（PM）and a ribbed meristem（RM）,where the CM is located at the top of the axillary meristem,which can form the PM and RM through the division of basal and lateral cells,the PM is located on both sides of the axillary meristem,and the RM is below the CM and on the inner side of the PM.The differences in cell volume and arrangement of the three cellular regions indicate that there are differences in cell division in the various parts of the meristem.At 0～4 h of decapitation,there was not anysignificant changes in the development of the three cell areas in the axillary meristem of Yunnan 87,while the number of cells in K326 increased significantly,and at 6 h,the number of CM cells of Yunyan 87 increased,and the cells of RM divided and grew into both sides,while the number of RM cells did not change significantly.The volume of CM and RM cells of K326 increased notably,and the number of cells in RM increased significantly,indicating that the development of the of Yunyan 87 axillary meristem was slower than that of K326.（2）SL inhibited the elongation of tobacco axillary buds by affecting endogenous hormone metabolism.,under GR24 treatment,the length of axillary buds significantly decreased by55.40%、33.03%、55.98%和29.30%compared with the control（CK）and TIS-108treatments on day 2 and 9,illustrating that GR24 could inhibit the elongation of axillary bud,but TIS-108 was able to effectively reduce the inhibitory effect of endogenous SL on tobacco axillary bud growth.HPLC-MS analysis showed that GR24 significantly reduced the levels of growth hormone（IAA）,trans-zeatin riboside（t ZR）,N6-（Δ2-isopentenyl）adenine（IPA）,gibberellin-7（GA₇）,jasmonic acid（JA）and jasmonic acid isoleucine（JA-Ile）in axillary buds on day 9 of treatment.In contrast,the levels of 1-aminocyclopropane-1-carboxylic acid（ACC）and gibberellin-1（GA₁）were increased by GR24 treatment.TIS-108 treatment significantly increased the levels of t ZR,SA,JA and JA-Ile but reduced the levels of ACC and GA₁on day9,GR24 notably increased the ratio of IAA to CTK and reduced the ratio of IAA to ABA,TIS-108 decreased IAA/CTK ratio but increased IAA/ABA,IAA/ACC and IAA/GAs ratios,the ratios between each endogenous hormone responded inversely to GR24 versus TIS-108.（3）Hormone-related genes and phenyl propane biosynthesis related genes may play an important regulatory role in the inhibition of tobacco axillary bud growth of SL.Expression analysis showed that the expression of IAA signaling genes AUXI1 and SAUR20,CTK biosynthesis gene IPT,CTK oxidase gene CKX1,GA oxidase gene GA2o X1,ABA signaling related gene PP2C,SL signaling negative regulator SMAX1 and SMAX1-LIKE4 were down-regulated by GR24 treatment from day 2 to 6.In contrast,the IAA metabolism gene GH3.1gene,the ETH biosynthesis gene ACO3 and its signaling gene ERF1,the SA signaling gene PR1,the SL biosynthesis gene D27 and its signaling related genes D14 and DAD2 were all down-regulated under the GR24 treatment.In addition,the expression of phenol propane biosynthesis gene HCT was up-regulated by GR24 treatment.The endogenous hormone-related genes in tobacco axillary buds may be involved in SL inhibition of tobacco axillary bud growth by regulating the level of endogenous hormone metabolism.（4）ABA can affect the expression of SL metabolism related genes.ABA significantly inhibited the expression of D27,SMAX1 and SMAX1-LIKE 4 genes,but D14 gene and its homologue DAD2 gene,both of which were induced by ABA treatment and highly expressed in tobacco axillary buds on day 3 to 5.